Terahertz (THz) technologies utilize electromagnetic radiation generally in the frequency range between 100 GHz and 10 THz (i.e., wavelengths of 3 mm to 30 μm, energies of 0.4 to 40 meV, or equivalent blackbody radiation temperatures of 5 K to 500 K). Terahertz technologies have many potential applications in diverse fields, including space and atmospheric sciences, molecular spectroscopy, remote sensing, biology, medical imaging, and communications.
However, beyond basic science, these applications in the terahertz region are relatively undeveloped. In particular, the generation and detection of electromagnetic fields at terahertz frequencies has been difficult. To date, active terahertz generators have only demonstrated relatively low power capability. Recently, terahertz sources based on quantum cascade lasers (QCLs) have produced relatively high power in a compact size. QCLs are the first semiconductor sources of terahertz radiation capable of average powers in a pulsed mode in excess of 250 mW at cryogenic temperatures. QCLs are unipolar semiconductor devices comprising complex layered heterostructures of two or more semiconductor alloys forming an active waveguide core, typically mounted on a metallic heat-sink. The complex QCL structure can be grown by molecular beam epitaxy (MBE). MBE enables accurate control of the sub-nanometer semiconductor layers with high reproducibility over hundreds of periodic layers.
Quantum cascade lasers rely on the emission from transitions between subbands in a quantum well. Light is produced in an active region by intersubband transitions of a single charge carrier (i.e., an electron) between two quantized levels in the conduction band. In a QCL biased at an operating voltage, a photon is emitted by an intrawell transition between an upper level and a lower level in an active region. To achieve population inversion for lasing, electrons must be injected rapidly into the upper level and then rapidly extracted from the lower level and tunnel into the upper level of the down-stream active region. To maximize the gain, tens to hundreds of these active regions can be cascaded together, enabling electrons that are recycled from one active region to the next to emit more than one photon per pass through the device, enabling high emission power. Because the energy difference between the two quantized levels is determined by the specific structure design (i.e., the quantum well and barrier widths), the laser can be band-structure engineered to emit at any wavelength within a broad spectral range. To minimize device loses and confine the terahertz radiation to the gain material, the active region can be inserted into a waveguide. Laser action requires that the gain be adequate to overcome device losses, primarily due to free-carrier losses in the waveguide and mirror losses. Many variations on this basic scheme have evolved.
However, the weak radiation output from passive and traditional active terahertz sources, the low photon energies of terahertz radiation, and high atmospheric attenuation due to molecular absorption (e.g., water vapor) frequently results in a weak received terahertz signal that may be difficult to distinguish from noise. Therefore, terahertz detection can also be difficult. Current terahertz detectors include both direct and heterodyne detectors.
Direct detectors generally directly convert the received power to a voltage or current that is proportional to the incoming power. Examples of direct detectors include rectifiers, bolometers, and pyroelectrics. A common direct detector uses antenna coupling to a small area Schottky diode that responds directly to the terahertz electric field. Detection depends on the nonlinear rectification properties of a metal-semiconductor junction. Advantages of the Schottky diode include a useful sensitivity over a large wavelength range, large instantaneous bandwidth, excellent performance at room temperature, and ease of fabrication.
For shorter wavelengths (i.e., frequencies above 1 THz), direct detectors generally have good responsivity and are sensitive to a broad band of frequencies. However, direct detectors generally provide no frequency discrimination, unless they are coupled with an external resonator or interferometer. Furthermore, they are sensitive to incoherent background noise and interference. Finally, direct directors are typically very slow, with 1 to 10 ms response times required to obtain an adequate signal-to-noise. Therefore, direct detectors have been used mainly for wideband applications, such as thermal imaging.
Heterodyned detection is desirable for some terahertz applications. Especially at low pressures such as a space environment, terahertz signatures of many molecules are very unique, enabling identification even with only a few spectral lines over a narrow spectral region. However, because the emission lines can be quite narrow and may be closely spaced, high resolution spectroscopy is desirable to take full advantage of terahertz discrimination capabilities. High-resolution heterodyne detection covering the frequency intervals of expected signatures is highly desirable for these applications. Further, particularly for weak signals, heterodyning can be used to coherently downconvert the terahertz signal to increase signal-to-noise by reducing bandwidth. The downconverted signal can then be post-amplified and processed using conventional microwave techniques.
Heterodyne mixers beat the signal RF frequency against a known local oscillator (LO) frequency to generate an intermediate frequency (IF) difference signal that is tunable through the LO frequency. The LO can have a fixed output power that is generally much greater than the power of the received RF signal. A nonlinear mixer produces an IF output power that is proportional to the product of the powers of the received RF signal and the LO signal. Mixers display good rejection of incoherent noise and interference. They are typically fast, with IF bandwidths of 0.1 to 10 GHz. Furthermore, narrowband detectors do not require additional frequency selective elements to analyze the spectrum of the incoming terahertz radiation as long as the received RF signal is within an IF bandwidth of the LO frequency. Therefore, heterodyne detectors have been used in narrow frequency band, high-resolution applications at lower terahertz frequencies, such as for molecular spectroscopy. Common mixers are field-type devices that have a strong quadratic nonlinearity.
Only within the last several years has the possibility of an all solid-state infrastructure for photonics at THz frequencies become a realistic possibility. This possibility has been primarily due to the invention and continued development of miniature semiconductor QCLs. Such QCLs are the only coherent solid-state source that can output the many milliwatts of average THz power necessary to transmit through the atmosphere and to supply sufficient LO power to THz diode receivers, with the goal of replacing the gas- and vacuum-tube THz sources most commonly used today. To date all reported THz photonic systems employing QCLs have used discrete source and detector components coupled via mechanically aligned free-space quasioptics, where coupling losses and system size pose significant impediments to practical use. To reach the same maturity level as existing infrared/visible photonics requires integration of solid-state THz sources, detectors, and auxiliary passive and active functionalities onto a compact, chip-based platform amenable to microfabrication methods.
Therefore, a fast solid-state terahertz radiation mixer is still needed to enable coherent detection for terahertz applications. In particular, a microelectronic-based integrated heterodyne terahertz transceiver is highly desirable for field-deployable applications. Such a transceiver requires the successful integration of both terahertz sources and detectors on a single chip, along with cooling, optics and control electronics, while maintaining high source power (e.g., greater than 10 mW), detection sensitivity, and operating temperatures, all in a compact, reliable, integrated package.